1. Field of the Invention
This invention relates to rheology-modified thermoplastic polyolefins, processes for making rheology-modified thermoplastic polyolefins and processes for shaping them into molded articles. In particular, this invention relates to rheology-modification of ethylene interpolymers such as ethylene/xcex1-olefin polymers.
2. Description of Related Art
Polymers and numerous additives are typically compounded into formulations which are then totally cross-linked for enhanced strength properties of the finished article. The starting polymer, prior to cross-linking, must have adequate performance properties such that it may be formulated or compounded with various additives and still maintain processability. For example, in a wire and cable coating operation, the composition must have xe2x80x9cgreen strengthxe2x80x9d, also known as xe2x80x9cmelt strengthxe2x80x9d, to remain on the wire after coating, and not sag or deform on the wire until the composition is cured. Otherwise the wire will have thin spots and the insulating value of the composition is lost. The composition must also undergo a final cure step and achieve good physical properties, such as tensile strength, elongation, and 100% modulus (stress at 100% strain). Typical curing occurs through use of peroxide or irradiation, and for polyethylene in general, the curing through crosslinking phenomenon is well documented (see, for example, Radiation Effects in Materials, A. Charlesby, editor, Pergamon Press, 1960). Polyethylene, especially heterogeneous linear low density polyethylene (LLDPE), when exposed to peroxide and/or radiation under proper conditions, forms gels as the molecular weight builds.
Usually the polymer selected to compatibilize all of the various components used in wire and cable coating operations is an elastomer such as ethylene/propylene rubber (EPR) or ethylene/propylene diene monomer terpolymer (EPDM). These types of very low density polymers is (i.e., polymers typically having a density less than 0.92 g/cm3) are relatively expensive (as compared with traditional linear low density polyethylene polymers) and contain a very high percentage by weight of comonomer(s) (e.g., propylene, dienes). Lowering the density of the polymer also increases the ability of the polymer to hold more filler and oil.
There have been a few recent announcements regarding new polymers which are said to be effective substitutes for EPR and EPDM. Union Carbide Chemicals and Plastics, Inc., announced in 1990 that they have developed a new cost effective class of polyolefins trademarked Flexomer(trademark) Polyolefins that could replace expensive EPR or EPDM rubbers. These new polyolefins are said to have bridged the gap between rubbers and polyethylene, having moduli between the two ranges.
While the development of new lower modulus polymers such as Flexomer(trademark) Polyolefins by Union Carbide or Exact(trademark) polymers by Exxon has aided the elastomeric formulation marketplace, there continues to be a need for other more advanced, cost-effective polymers which can ultimately be fully cross-linked to form a polymer aggregate such that the bulk polymer is a covalently bonded network of polymer chains, but which also have good physical properties and processability prior such to complete cross-linking.
Others have attempted to modify polyolefins in various ways to try to achieve such goals. For example, in Chemical Modification of Linear Low Density Polyethylene, by T. K. Su, R. G. Shaw, P. J. Canterino, E. A. Colombo and T. H. Kwack, published in ANTEC ""87 SPE Technical Papers, vol. 33, pp. 1271-1275, linear low density polyethylene (LLDPE) was crosslinked using peroxide free-radical initiation. This modification is said to result in chemically modified LLDPE without creating gels. However, Su et al. also report that peroxide modification of LLDPE results in higher apparent viscosity throughout the range of shear rate (see FIG. 2 of Su et al.). This change in viscosity indicates growing molecular weight as a result of the peroxide modification and results in modified LLDPE which does not have the same processability as the unmodified LLDPE, especially in the high shear range.
In PCT/GB85/00142 (published as WO 85/04664) (xe2x80x9cPCT ""142xe2x80x9d herein), LLDPE is treated to enhance the polymers"" suitability for extrusion conversion into hollow articles (e.g., tubes, sheathing, and wire and cable insulators). PCT ""142 states that treating LLDPE having a melt index over 3 g/10 minutes with xe2x80x9cmoderate quantities of peroxide does not bring about an adequate broadening of molecular weight distribution and may lead to treated LLDPE""s whose mechanical properties are unsatisfactory.xe2x80x9d Further, these treated LLDPE""s are said to produce finished extruded articles which have a xe2x80x9cnon-uniform wall and a rough surfacexe2x80x9d as a result of xe2x80x9cshark-skinxe2x80x9d melt fracture. PCT ""142 allegedly solves the difficulty by using thermo-mechanical treatment of the LLDPE in a molten state. The treatment involves introducing LLDPE having a density of 0.9 to 0.935 g/cm3 and a melt index over 3 dg/minute as a powder into a thermomechanical apparatus of an extruder while simultaneously introducing an organic peroxide at a level of over 0.05% and less than 1% (by weight of the polymer).
U.S. Pat. No. 4,598,128 (Randall et al.) describes ethylene polymer compositions being a blend of a first and second ethylene polymer. The second ethylene polymer is characterized by molecules having long chain Y-branches. Both polyethylenes can be made using the high pressure process (producing homopolymer low density polyethylene (LDPE)) or in a low pressure process (producing linear polyethylene having essentially no long chain branching). The blend can be prepared by using an extrusion process in which a portion of the polyethylene is irradiated and both the irradiated and non-irradiated polymers subsequently melt blended. The long chain Y-branched polymer is said to have a broad molecular weight distribution. The resultant blended composition is also said to have altered rheological properties without significantly increasing the molecular weight of the polymers. The compositions are said to be useful for coatings and production of shaped and molded articles (e.g., pipes, gas tanks and other molded auto parts).
While there have been several attempts at increasing the processability of linear heterogeneously branched polyethylene through use of irradiation, there continues to be a need for cost effective modification of polyethylene such that the resultant modified polymer is still useful for thermoplastic molding processes. In particular, there is a need for polyolefins having one or more improved processing characteristics such as higher zero shear viscosity, low high shear viscosities, improved melt flow (I10/I2) properties, improved critical shear rate at onset of surface melt fracture, improved critical shear stress at onset of gross melt fracture, improved rheological processing index (PI), improved melt strength, higher green strength, greater filler/plasticizer/oil loading capabilities, and/or improved peroxide cure efficiency, while maintaining or improving physical properties such as tensile strength, impact strength, modulus of elasticity and relaxation time. In blown film processes high bubble stability, particularly combined with high polymer throughput, is a particularly desirable objective and in cast film and extrusion molding processes the ability to increase or maintain the polymer throughput rate and/or reduce or maintain extruder back pressure while improving draw down and/or reducing neck in is particularly desired.
These and other desired goals are satisfied by ethylene polymers selected and modified according to the present invention.
One aspect of this invention is directed to a rheology-modified ethylene polymer having less than 0.5 wt % gel as measured via ASTM D2765, Procedure A, a Composition Distribution Branch Index (CBDI) greater than 50 percent and a molecular weight distribution less than 4.0, which is characterizable by one of the following equations:
Zxe2x89xa6(log xcex70.1xe2x88x92log xcex7100)/log xcex7100xe2x80x83xe2x80x83(I)
log xcfx840=mxc2x7log (xcex70)xe2x88x92bxe2x80x83xe2x80x83(II)
Kxe2x89xa6MS150 Cxe2x88x9272.079xc3x97(log Mw)2+666.28xc3x97(log Mw)xe2x88x921539.5xe2x80x83xe2x80x83(III)
wherein xcex70 is the zero shear rate viscosity of the polymer, xcex70.1 is the viscosity of the polymer measured at 190 C and a shear rate of 0.1 radians/second, xcex7100 is the viscosity of the polymer at a shear rate of 100 radian/second, xcfx840 is the relaxation time of the polymer, Z, also referred to herein as the log viscosity ratio, is a number having a value of 0.30, m is a number having a value greater than or equal to 1.070, b is a number having a value less than or equal to 5.950, K, also referred to herein as the melt strength improvement constant, is a number equal to 0.50, MS150 C is the melt strength of the rheology-modified polymer in centiNewtons (cN) at 150 C and Mw is the weight average molecular weight of the rheology-modified polymer as measured via gel permeation chromatography (GPC).
Another aspect of this invention is directed to a process for improving the processability of a thermoplastic ethylene polymer comprising treating at least one thermoplastic ethylene polymer having a molecular weight distribution less than 3.00 and a CBDI greater than 50 percent with a crosslinking agent in an amount less than the amount which would cause greater than or equal to 0.5 wt % gel formation under melt processing conditions wherein the process satisfies the condition:
log xcex70.1mxe2x89xa7log xcex70.1v+xxe2x80x83xe2x80x83(IV) and
log xcex7100mxe2x89xa6log xcex7100v+yxe2x80x83xe2x80x83(V)
wherein xcex70.1m and xcex7100m are the viscosities of the modified polymer in poise measured at 190 C and shear rates of 0.1 and 100 radian/second, respectively, xcex70.1v and xcex7100v are the viscosities of the unmodified polymer (i.e., the xe2x80x9cvirginxe2x80x9d polymer) in poise measured at 190 C and shear rates of 0.1 and 100 radian/second, respectively, x is a number having a value of 0.50 and y is a number having a value of 0.10;
log xcfx840mxe2x89xa7log xcfx840v+0.1xe2x80x83xe2x80x83(VI)
wherein log xcfx840m and log xcex70v are log relaxation times of the rheology-modified polymer and the polymer prior to modification, respectively; or
MSmxe2x89xa7MSv+0.5 cNxe2x80x83xe2x80x83(VII)
wherein MSm and MSv are melt strengths in cN at 150 C of the rheology-modified polymer and the same polymer prior to modification, respectively.
Another aspect of this invention is directed to an improved process for making a molded article comprising:
a) treating at least one thermoplastic ethylene polymer having a molecular weight distribution less than 3.00 and a CBDI greater than 50 percent with a crosslinking agent;
(b) heating the treated polymer to a temperature suitable for melt processing;
(c) melt processing the heated polymer;
(d) forming the melt processed polymer into a shape and
(e) allowing the shaped polymer to cool,
wherein the amount of crosslinking agent used in step (a) is less than that which would cause greater than or equal to 0.5 wt % gel formation under the melt processing conditions of step (c) and yet sufficient to satisfy the condition:
log xcex70.1mxe2x89xa7log xcex70.1v+xxe2x80x83xe2x80x83(IV) and
log xcex7100mxe2x89xa6log xcex7100v+yxe2x80x83xe2x80x83(V)
wherein xcex70.1m and xcex7100m are the viscosities of the modified polymer in poise measured at 190 C and shear rates of 0.1 and 100 radian/second, respectively, xcex70.1v and xcex7100v are the viscosities of the unmodified polymer (i.e., the xe2x80x9cvirginxe2x80x9d polymer) in poise measured at 190 C and shear rates of 0.1 and 100 radian/second, respectively, x is a number having a value of 0.50 and y is a number having a value of 0.10;
log xcfx840mxe2x89xa7log xcfx840v+0.1xe2x80x83xe2x80x83(VI)
wherein log xcfx840m and log xcfx840v are log relaxation times of the rheology-modified polymer and the polymer prior to modification, respectively; or
MSmxe2x89xa7MSv+0.5 cNxe2x80x83xe2x80x83(VII)
wherein MSm and MSv are melt strengths in cN at 150 C of the rheology-modified polymer and the same polymer prior to modification, respectively.
Yet another aspect of the present invention is directed to improved intermediates for making molded articles comprising a thermoplastic ethylene polymer having a CBDI greater than 50 percent treated with a crosslinking agent in an amount less than sufficient to cause the formation of 0.5 wt % or more gel under melt processing conditions and yet sufficient to satisfy the condition:
log xcex70.1mxe2x89xa7log xcex70.1v+xxe2x80x83xe2x80x83(IV) and
log xcex7100mxe2x89xa6log xcex7100v+yxe2x80x83xe2x80x83(V)
wherein xcex70.1m and xcex7100m are the viscosities of the modified polymer in poise measured at 190 C and shear rates of 0.1 and 100 radian/second, respectively, xcex70.1v and xcex7100v are the viscosities of the unmodified polymer (i.e., the xe2x80x9cvirginxe2x80x9d polymer) in poise measured at 190 C and shear rates of 0.1 and 100 radian/second, respectively, x is a number having a value of 0.50 and y is a number having a value of 0.10;
log xcfx840mxe2x89xa7log xcfx840v+0.1xe2x80x83xe2x80x83(VI)
wherein log xcfx840m and log xcfx840v are log relaxation times of the rheology-modified is polymer and the polymer prior to modification, respectively; or
MSmxe2x89xa7MSv+0.5 cNxe2x80x83xe2x80x83(VII)
wherein MSm and MSv are melt strengths measured in cN at 150 C of the rheology-modified polymer and the same polymer prior to modification, respectively.
Yet another aspect of this invention is a method of using the intermediates described above in a method for making an article comprising a polymer.